1. Field of the Invention
The invention relates in general to white-light interferometry. In particular, it relates to a new approach for producing microscope-like color images from interferometric data.
2. Description of the Related Art
The desirability of high-precision color images has been increasingly felt in various arts because of the structural and compositional information provided by color. For example, the boundary between different materials can be observed more readily if a sample is imaged sharply and in color. To that end, bright-field white-light microscope images can be improved by scanning through focus the sample surface to obtain the sharpest possible local in-focus images which are then combined to produce an improved color image of the surface. In addition, the scan data can be used to determine the vertical position of each local focal plane, which in turn also enables the three-dimensional color image representation of the sample.
While images produced by bright-field microscopy have the advantage of true color representation, they are limited with regard to the precision of the height measurements derived from through-focus scanning. Therefore, interferometric approaches, which can determine heights with nanometer precision, have gained much acceptance where accurate three-dimensional representations are required. The problem, however, has been that the irradiance data provided by interferometric measurements is not directly suitable for color images because of the effects of interference and the contribution of reference-mirror light to the wavefront detected by the camera. In fact, until recently, the main objective of interferometric measurements was simply the precise three-dimensional mapping of samples and color was typically used arbitrarily only to highlight structural variations, not to show true color.
Imaging of true color has now become an important aspect of interferometric profiling, especially in the fields of biology and pathology, where color as well as structure provides a great deal of visual information to the trained observer. Accordingly, much effort has been devoted to obtaining color images that represent the sample with the high precision afforded by interferometric measurements. For example, one approach has been to provide a 3-D interferometric objective and a bright-field objective in the same instrument to produce both height and color data through independent measurements that are then combined to obtain a high-precision 3-D color image. However, this approach requires two separate measurements, a process that lengthens the data acquisition time beyond what may be acceptable for certain applications.
In Japanese Patent Application No. 2010-112865, Yamauchi Takashi describes an interferometric color-imaging approach based on the usage of either a monochromatic or an RGB (red-green-blue) color camera. Three light sources (RGB, generated either independently or through filtered white light) are sequentially activated at three separate scan times at each interferometric acquisition frame so as to generate simultaneously interferometric data for each color from which the relative height of each pixel is determined. The interference effect is then removed from the data to provide true RGB color information that is combined with the height data to produce a full-color interferometric 3-D image. Since both height and color are generated by the same measurement, the procedure is precise and faster than what could be attained by combining microscope and interferometer data.
A similar approach is described by T. Machleidt et al. in a publication entitled “Application of Color Cameras for 3D Surface Measurements with White Light Interferometers” presented by the German company GBS GmbH at the 14th SpectroNet Collaboration Forum in Jena Germany. Using individual RGB interferometric data generated with white light detected with a color camera, the sets of color modulation data recorded for each pixel (in so-called Bayer pattern) are spectrally reconstructed and correlated to derive true-color approximations. These are then combined with the height measurements to produce full-color 3-D representations of the sample surface.
The present invention addresses the same problem with a simplified approach for producing in a rapid and computationally efficient manner a full-color interferometric image of a sample surface. The procedure is based on the mathematical representation of the irradiance produced by interferometry.